Parallax in Survey Telescopes – Correct Procedure to Remove It Completely To eliminate parallax so that the image of the object and the cross-hairs lie in the same focal plane, which focusing steps must be correctly performed?

Introduction:
Parallax causes apparent relative motion between the target image and the cross-hairs when the observer’s eye shifts. It leads to pointing errors in angle and staff readings. The remedy requires a specific focusing sequence to make both images coincide in one plane.

Given Data / Assumptions:

• Cross-hair diaphragm position is fixed inside the eyepiece assembly.
• Eyepiece focuses the observer’s eye on the cross-hairs (forming a virtual image at infinity).
• Objective forms the target image at the diaphragm plane when correctly focused.

Concept / Approach:

First, remove ocular accommodation effects by focusing the eyepiece until the cross-hairs appear razor sharp against a bright background. This sets the eye at the correct optical condition. Next, use the objective focusing to bring the external object’s image exactly onto the cross-hair plane. When both coincide, shifting the eye causes no relative motion, indicating zero parallax.

Step-by-Step Solution:

Verification / Alternative check:

Standard theodolite and level manuals prescribe the same two-step focusing order for parallax removal.

Why Other Options Are Wrong:

Eyepiece only or objective only will not ensure both images share a plane; rotating the telescope does nothing to parallax.

Common Pitfalls:

Focusing the objective first; relying on eye accommodation which reintroduces parallax for other users.

Final Answer:

Focus the eyepiece first for sharp cross-hairs, then focus the objective on the target